Operating heads of the above indicated type are to be used on machines in which the operating head is movable along one or more axes by means of electronically controlled motor means in order to perform cutting operations on pieces arranged on the machine table. In machines of this type the addition of the abrasive agent to the high pressure water jet enables cutting operations to be performed also on materials of medium and high hardness, and in general on all those materials for which cutting by a pure water jet is not possible.
The invention relates to a water-jet operating head of the known type including:                a head main portion, intended to be mounted on an operating machine, and defining an axial passage for feeding a high pressure water flow,        a nozzle carrying support which is removably connected to the head main portion,        a jet focusing nozzle, for emitting a high pressure hydro-abrasive jet, mounted on said nozzle-carrying support and defining an ejection passage which is arranged downstream of said feeding passage and is in communication therewith,        a primary nozzle carried by said nozzle-carrying support upstream of the focusing nozzle and having a body with an orifice of predetermined diameter interposed in the communication between the water flow feeding passage and the hydro-abrasive jet ejecting passage, and        at least one connecting element carried by said head, through which an abrasive agent can be fed into the high pressure water flow at a mixing chamber defined within the nozzle-carrying support, between the primary nozzle and the focusing nozzle.        
Machines provided with operating heads of the above indicated type have been produced and marketed for some time by the same Applicant.
A problem which is encountered in the operating heads of the above described type lies in the wear of the components inside the head through which the high pressure jet flows, as well as in the wear of some parts in which also the added abrasive flows. This causes an enlargement of those passages, with a resulting decrease of effectiveness and precision in the cutting operation. The parts which are more subject to wear are the above mentioned focusing nozzle and the primary nozzle. The focusing nozzle has the function of focusing a coherent jet of water and abrasive which is projected against the piece surface to cut the material being worked. The primary nozzle, along with its orifice of predetermined reduced diameter, has the function of accelerating the water jet and is arranged and self-centered inside the nozzle-carrying support. Between the primary nozzle and the focusing nozzle there is interposed a mixing chamber where the water jet is added with abrasive sand. The primary nozzle, the nozzle-carrying support along with the mixing chamber and the focusing nozzle are partly or completely preassembled with each other.
During use of the machine, it is necessary to periodically replace the above mentioned parts of the head which are subject to wear in order to always ensure the requested effectiveness and precision of the cutting operations.
Water-jet heads of various types are known, for example in U.S. Pat. No. 4,555,872. In this known solution, the operations for replacing the parts subject to wear and particularly the nozzle, are time-consuming, cumbersome and not adapted to be automated.